The present invention relates generally to mixing systems for use in enclosed vessels, such as rigid or flexible enclosures, or open vessels, such as pond systems, which may serve as reactors, bioreactors or photobioreactors. Systems in accordance with the present invention may be used to cultivate algae and other microorganisms in water for purposes such as producing biofuels, bulk chemicals, pharmaceutical compounds or other products or treating wastewater.
High density, high pigment aqueous algae cultures require mixing to evenly distribute nutrients to microorganisms in the culture and to ensure that the microorganisms in the culture are cyclically exposed to light needed for photosynthesis. One of the key challenges for commercial-scale mixing systems is to minimize the use of energy and capital expense while providing optimal production conditions.
Large, open pond systems typically use large paddle wheel mixers to move water around a raceway, but paddle wheel mixers are inefficient and require significant energy inputs, which may be cost-prohibitive for use in cultivation of microorganisms for the production of biofuel or other commodities. In addition, paddle wheel mixers are designed to move water in a path of horizontal flow and do not effectively move algae in a vertical plane, which is needed to ensure even exposure of the algae to light at the surface of the aqueous culture.
In “Biotechnology of Algal Biomass Production: A Review of Systems for Outdoor Mass Culture,” Journal of Applied Phycology 5: 593-604 (1993), Chaumont reviews mixing techniques proposed for use with algae cultures, including forcing culture through a slit in a board dragged through an open pond; “a mixing system consisting of a continuous flume containing arrays of foils similar in design to segments of airplane wings”; airlift; injectors; propellers; pump and gravity flow devices using natural energy sources; open pond loop “raceways” incorporating paddlewheel stirring devices; and sloped ponds and other cultivation units having parallel troughs or baffles, for example.
In “Photobioreactors for Mass Cultivation of Algae,” Bioresource Technology 99: 4021-4028 (2008), Ugwu et al. note that inefficient stirring mechanisms in open cultivation systems yield poor mass transfer rates that result in low biomass productivity.
Vertical photobioreactor systems use pumps, blowers or compressed air to introduce rising air bubbles and produce turbulent fluid motion in the aqueous algal culture for the purpose of mixing. Horizontal photobioreactor systems typically use pumps to circulate the culture and create turbulence in the aqueous algae culture to provide mixing.
Ugwu et al. (“Photobioreactors for Mass Cultivation of Algae”) describe the use of air-pump, bubble column and airlift systems to mix cultures in tubular and vertical-column photobioreactors.
The effects and performance of mixing in vessels such as bioreactors have also been investigated for numerous configurations of other mixing elements, such as the combination of radial impellers with axial up-pumping hydrofoils (Vrabel et al., “Mixing in Large-Scale Vessels Stirred With Multiple Radial or Radial and Axial Up-Pumping Impellers: Modelling and Measurements,” Chemical Engineering Science, Vol. 55, No. 23: 5881-5896 (2000)); a rotating impeller in combination with glass tubes acting as baffle plates (Ogbanna et al., “A Novel Internally Illuminated Stirred Tank Photobioreactor for Large-Scale Cultivation of Photosynthetic Cells,” Journal of Fermentation and Bioengineering, Vol. 82, No. 1:61-67 (1996)); up-pumping impellers (Nienow et al., “The Versatility of Up-Pumping Hydrofoil Agitators,” Chemical Engineering Research and Design, Vol. 82, No. 9: 1073-1081 (2004)); axial and mixed dual-impeller systems (Bouaifi et al., “Power Consumption, Mixing Time and Homogenisation Energy in Dual-Impeller Agitated Gas-Liquid Reactors,” Chemical Engineering and Processing, Vol. 40, No. 2: 87-95 (2001)); the combination of airlift with hydrofoil impellers (Chisti et al., “Oxygen Transfer and Mixing in Mechanically Agitated Airlift Bioreactors,” Biochemical Engineering Journal, Vol. 10, No. 2: 143-153 (2002)); and turbines, down-pumping hydrofoils and up-pumping hydrofoils (Boon et al., “Comparing a Range of Impellers for ‘Stirring as Foam Disruption’,” Biochemical Engineering Journal, Vol. 10, No. 3: 183-195 (2002)).
In “A Simple Algal Production System Designed to Utilize the Flashing Light Effect,” Biotechnology and Bioengineering, Vol. XXV: 2319-2335 (1983) and in “High Algal Production Rates Achieved in a Shallow Outdoor Flume,” Biotechnology and Bioengineering, Vol. XXVIII: 191-197 (1986), Laws et al. describe gains in solar energy conversion efficiency and algae production yielded by emplacing arrays of foils similar in design to airplane wings to create vortices and systematic mixing in an algal culture flume.
Many of these methods provide mixing regimes for high-density algae cultures but consume too much energy to be cost effective in the production of biofuel, bulk chemicals or other commodities on an industrial scale. The required energy inputs for such methods and configurations exceed the energy yield that can be produced by the algae culture in the form of, for example, biofuel. Accordingly, a need exists for a mixing system that provides sufficient mixing and gas transfer for optimal production of biofuel and other materials while maintaining acceptable energy consumption in the context of operating costs for the reactor system and minimizing capital expense.
In addition, a need exists to provide effective mixing and gas transfer in vessels such as bioreactors and photobioreactors while maintaining structural integrity of the vessel, minimizing risk of contamination of the contents of the vessel, minimizing exposure of pumps and other mixing drive system components to corrosive agents in the vessel and facilitating ease of maintaining the drive system components. Various mixing apparatuses rely on the use of rotary impellers and similar elements that are not physically connected to a drive motor but instead are driven by magnetic coupling.
U.S. Pat. No. 7,824,904 (Dimanshteyn for “Photobioreactors for Production of Algae and Methods Therefor”) discloses mixing a liquid microbial culture using a rotary or oscillatory system comprising one or more motors, one or more shafts connected to the one or more motors and a plurality of mixing blades attached to the one or more shafts.
U.S. Pat. Appl. Pub. No. 2009/0035856 (Galliher et al. for “Continuous Perfusion Bioreactor System”) discloses vessels such as a disposable, collapsible bag having an integrated magnetically-driven rotating impeller that provides mixing for cell culture, cell containment, bioreactor and/or pharmaceutical manufacturing systems.
U.S. Pat. Appl. Pub. No. 2009/0130757 (Terentiev for “Bioreactor With Mixer and Sparger”) discloses a bioreactor that comprises an impeller positioned within an interior compartment of the vessel that is rotated by way of a magnetic coupling.
U.S. Pat. Appl. Pub. No. 2011/0003366 (Zeikus for “Methods of Using Pneumatic Bioreactors”) discloses a pneumatic bioreactor containing a fluid to be mixed that includes a floating impeller that rises in the fluid as gas bubbles carry it upward to the surface and falls when the gas is then vented, wherein the mixing speed is controlled with electromagnets in the vessel acting upon magnetic material in the impeller or its guides.
PCT Published Patent Application WO 2005/121310 (Johnson et al. for “Creation of Shear in a Reactor”) discloses the use of a applying a magnetic field to a magnetically-activated element to generate shear in a liquid sample.
U.S. Pat. Appl. Pub. No. 2009/0219780 (Castillo et al. for “Mixing System Including a Flexible Bag, Specific Flexible Bag and Locating System for the Mixing System”) discloses a mixing system comprising a flexible bag with a rotary magnetic impeller and an alignment facilitation device adapted to facilitate alignment between the magnetic impeller and a magnetic driver located external to the system.
In “Design, Construction and Testing of Pilot Scale Photobioreactor Subsystems,” Master of Science (MS) Thesis, Ohio University, Mechanical Engineering (Engineering and Technology), 2008, Mears describes the work of Tsygankov (2001) involving a coaxial cylinder reactor in which two coaxial tubes are placed one inside the other with algae fluid located in the annular space between the surfaces of both tubes. Mears further describes the reactor of Tsygankov incorporating a ferromagnetic ring in the section containing the algae and applying a magnetic field to move the ring back and forth, mixing the algae liquid.
In “Microbioreactors for Bioprocess Development,” Journal of the Association for Laboratory Automation, Vol. 12, No. 3: 143-151 (2007), Zhang et al. describe the use of a magnetic stir bar to mix a microbial solution in a cylindrical reactor chamber.
A need exists to incorporate a magnetic coupling drive system with a mixing configuration that is effective in a photobioreactor while maintaining the structural integrity of the photobioreactor and ability to service the components of the drive system without compromising the algae culture therein.
The above discussion includes both information known to the art prior to the filing date and information forming part of the present inventive disclosure. Inclusion of any statement in this section, whether as a characterization of a published reference or in a discussion of technical problems and their solutions, is not to be taken as an admission that such statement is prior art.